Modern cellular radio communication systems, like the 3GPP LTE system (3GPP=Third Generation Partnership Project, LTE=Long-Term Evolution), rely on MIMO antenna techniques (MIMO=Multiple Input Multiple Output) in order to achieve high spectral efficiency.
Downlink CoMP also known as network MIMO is an effective transmission scheme for improving an overall performance of the radio communication systems. Thereby, the overall data rate can be increased without requiring additional frequency spectrum or without requiring an increase of transmit power. Especially users that are located at the border between two radio cells or two radio sectors can benefit from downlink CoMP transmissions.
Downlink CoMP is a transmission of downlink data from a so-called multipoint transmitter of a radio access network to one or several so-called single point receivers such as one or several mobile stations. The multipoint transmitter is provided by two or more antenna systems of the radio access network, which are arranged at different locations. The two antenna systems may be for example two base stations or two RRHs (RRH=Remote Radio Head). The single point receiver is connected only to a single antenna system, which may contain one antenna element or multiple antenna elements.
The downlink CoMP transmission scheme and a corresponding interference alignment require accurate channel state information at the multipoint transmitter for being able to determine suitable downlink CoMP transmission parameters in such a way, that inter-cell interference via spatial processing (e.g. precoding) is suppressed and useful signal contributions coherently add-up at the single point receiver.
In case of downlink CoMP transmission in a so-called TDD transmission mode (TDD=Time Division Duplex), a reception of uplink sounding signals at receivers co-located with the multipoint transmitter may be applied and reused for getting knowledge of channel states of the downlink channels from the multipoint transmitter to the single point receiver. But in case of downlink CoMP transmission in a so-called FDD transmission mode (FDD=Frequency Division Duplex), a full channel reciprocity is not available at the multipoint transmitter due to a so-called duplex distance between a downlink frequency range used for the downlink CoMP transmission and an uplink frequency range used for an uplink transmission from the mobile station to a single antenna system or to several antenna systems (also called uplink CoMP transmission) of the radio access network. In such a case channel characteristics or conditions such as noise or fast fading of an uplink channel are quite different to channel characteristics or conditions of a downlink channel and therefore cannot be reused for a determination of suitable transmission parameters for the downlink CoMP transmission. Therefore, channel conditions of the downlink CoMP transmission have to be measured at the mobile station and have to be fed back to the radio access network via uplink signaling.
The uplink signaling is performed individual by each mobile station, which means that each mobile station may report channel conditions for a so-called reporting cluster of radio cells, which may be predefined by a network node such as a master base station for the downlink CoMP transmission. The master base station uses the uplink signaling from multiple mobile stations for calculating optimum downlink CoMP transmission parameters for downlink transmissions to the multiple mobile stations.